Meteorites
Geoff Steel, 13th October 2010
Introduction
Most of us have seen meteors, or ‘shooting stars’. They are small particles, about the size of sand grains, which enter the earth’s atmosphere and burn up due to friction. Occasionally a much larger object, perhaps the size of a football, may come in. It gives a spectacular display. Lighting up the sky it can be brighter than the full moon and leave a trail lasting for several minutes, followed by a sonic boom which sounds like thunder. It falls to the ground as a meteorite.
Meteorites have been collected and studied for hundreds of years. There were many fanciful ideas about their origins. Serious scientific study began in 1794 when Ernst Chladni established that they were indeed extraterrestrial in origin, as opposed to volcanic. Invention of the petrographic microscope in the 1860s provided the basis for detailed analysis but studies were always limited by the great rarity of meteorites, there being a total of only about two thousand in all the worlds museums until the 1960s.
Then in 1969 came a remarkable discovery. A group of Japanese glaciologists were working in the Yamato Mountains in Antarctica. One of them found a meteorite. They looked around and found eight more on the same patch of ice. Not knowing the significance of their discovery they simply packed them away for later study. On return to Japan the samples caused amazement. What mechanism had concentrated them into one place? And could there be more? Both Japan and America organised meteorite hunting expeditions.
Antarctica is a vast dome of ice. It flows outwards from the centre carrying with it any meteorites which happen to fall, the flow taking thousands of years. It is a treasure trove of meteorites but almost all are lost as the glaciers break away at the edges into icebergs. However if a glacier is impeded, say by a rock ridge underneath, then some ice becomes stagnant and its surface is ablated away by the wind. Any solid objects are left on top. Such a place is called a meteorite stranding surface and it was one of these that the Japanese had accidentally discovered.
The Antarctic expeditions have been greatly successful. We now have over thirty thousand meteorites. Their study has opened up an entirely new understanding of the early history of our solar system and the formation of the planets.
Meteorite Classification
Three kinds of meteorites have long been recognised: stones, stony-irons and irons. Stones are themselves divided into two different types. To understand the full classification it is helpful to start in familiar territory. Figure 1 shows a cross section of a planet like the Earth. At the centre is an iron core surrounded by a mantle made almost entirely of olivine. On top is a thin crust, mainly of basalt (olivine + pyroxene + plagioclase). The planet is described as ‘differentiated’, meaning that at some time in its history it was completely molten, allowing the layers to separate like oil and water.
Figure 1 Origin of the differentiated meteorites.
It seems that the early solar system had many such planets, most of them far smaller than the Earth. Sometimes there were collisions. Impacts caused fragmentation into small pieces, which we now collect as meteorites. They are described as ‘differentiated’ meteorites due to the nature of their parent bodies. The figure shows irons to originate from the core, stony-irons from the core/mantle boundary, and stones from the crust. These particular stones are called achondrites.
The above description raises an obvious question: What were such planets originally made of before they differentiated?
In fact the differentiated meteorites are all quite rare. About 90% of all meteorites are remains of the original material. They are stones called chondrites. The name refers to their primary constituent which is tiny spheres about 1mm in diameter. The spheres are called chondrules. Figure 2 shows a typical example.
Figure 2 Section of a chondrite
Individual chondrules are composed mainly of olivine and/or pyroxene. Laboratory experiments show that they formed by sudden heating of dust particles, followed by rapid cooling. They are set in a dark matrix of olivine and pyroxene, with glass-like fragments of plagioclase and traces of carbon. The matrix was originally dust and has been compressed to a solid. Some chondrites also contain iron particles and irregular inclusions of calcium and aluminium minerals.
Early investigators identified what appeared to be a metamorphic sequence within the chondrites. Some appeared unaltered, while others showed increasing effects of heating. The grades were numbered 1 to 6. The same sequence is still used today, however is now known that grade 3 actually represents pristine chondritic material, with increasing thermal metamorphism to grades 4, 5 and 6, but aqueous alteration producing grades 2 and 1. Classification is summarised in figure 3.
Figure 3 Classification of the chondrites (grey = exists)
It is only in the carbonaceous chondrites that aqueous alteration is found. The presence of water indicates formation in a cold region and the same meteorites also contain organic compounds such as amino acids, which would be destroyed by heating. The amino acids are a mixture of left-handed and right-handed forms, showing that they were not biologically produced, but there has been much speculation about their significance as a supply of organic material to early life forms on Earth. Carbonaceous chondrites are divided into several groups based on chemical composition.
By far the most common meteorites are the ‘ordinary’ chondrites. They are divided into groups H, L and LL based on their iron content (high, low and very low). All were formed at temperatures too high to contain water, and most show thermal metamorphism.
Finally there are the enstatite chondrites. Enstatite is magnesium silicate (one of the pyroxene group of silicates) and only forms at high temperatures. There is a division between high and low iron contents, EH and EL respectively, with thermal metamorphism common in both types.
The Asteroid Belt
Some meteorites have been observed accurately enough when falling to calculate their original orbits. All have come from the asteroid belt. Hence it is that part of the solar system for which they provide the most detailed information.
Observations of young stars show that they form at the centre of flat spinning disks of gas and dust. In the outer part of the disk the clumps of dust attract by gravity, forming many small planet-like bodies called planetesimals, and the process continues until all the dust is used up. The planetesimals themselves accrete to form planets. In our solar system the fifth planet, Jupiter, became so large that it prevented formation of any other planet nearby, any large object being torn apart by tidal forces. The result was the asteroid belt. It is the region between Mars and Jupiter where there are many small planetesimals (asteroids) that have never formed a planet. They are remnants of the original solar disk and we are fortunate to have meteorites as samples. The process is summarised in Figure 4.
Figure 4 Asteroid formation
In some parts of the solar disk the dust was heated above its melting point, forming the small spherical chondrules. The exact mechanism of heating is not known and is a subject of great debate among meteorite specialists. After cooling the chondrules were mixed back together with dust, perhaps by the early solar wind, and accreted into planetesimals. Those which accreted close to the sun included high temperature minerals like enstatite, while further away it was cold enough for water.
Older text books describe accretion as taking tens or even hundreds of millions of years. Meteorite studies show that it was actually a much faster process. The evidence comes from radioactive decay of 26Al. Flat spinning disks from which stars originate are themselves formed by compression of gas clouds and in our case compression was caused by a nearby supernova. It produced the heavy elements up to uranium along with 26Al which has a half-life of just 0.73Myr.
Figure 4 shows that immediately after accretion a planetesimal would have uniform composition of pristine chondritic material, in this example shown as L3. But decay of 26Al caused internal heating. The result was metamorphism to successively higher grades towards the centre, leading to an ‘onion skin’ composition. If the planetesimal was large enough (greater than about 10km diameter) the process continued to the point of complete melting, resulting in a differentiated body. Internal heating would be effective only within the first few half-lives of 26Al. Therefore the whole process of accretion, metamorphism and differentiation must have been completed within about three million years.
Most carbonaceous chondrites do not show the effects of heating. They contained the same concentration of 26Al but they also contain water. The latter has such high heat capacity that energy released by radioactivity caused hardly any temperature rise.
Figure 4 also shows that a planetesimal may be fragmented by a collision but reformed into a ‘rubble pile’. Evidence for this comes from the many chondrites which are breccias made up of pieces with varying metamorphic grades. Further evidence comes from recent fly-bys of asteroids by the Galileo and NEAR spacecraft. Chondrites have a density of around 3.0 kg/m3 but asteroids Ida and Mathilde were measured at 2.6 and 1.3 respectively, indicating significant internal voids.
Spectroscopic studies of the asteroids show a remarkable correlation with meteorite composition. Asteroids on the inner edge of the belt, close to the orbit of Mars, have spectra which match the enstatite chondrites, known to have formed at high temperature. At the centre the spectra match ordinary chondrites, while at the outer edge, in the cold region towards Jupiter’s orbit, the spectra match carbonaceous chondrites.
HEDs, SNCs and Lunaites
Dating by Rb/Sr shows that almost all meteorites have the same age: 4.56Byr. This fixes very precisely the formation of the solar system. However a very small number are newer. They are all achondrites. Their composition is similar to basalt and their age indicates parent bodies large enough to retain sufficient heat for volcanism long after differentiation.
The oldest group are the HEDs at 4.4Byr. The initials stand for Howardites, Eucrites and Diogenites, all crustal materials. Spectral studies show that they match exactly with Vesta, one of the largest of the asteroids. Vesta’s orbit is not in a position where material could easily reach Earth but there are several small asteroids called Vestoids, with identical spectra, which appear to be pieces knocked off it. They do have suitable orbits and are believed to be the source of the HEDs.
At the newest age are the SNCs, pronounced ‘snicks’. Their date is only 1.3Byr which implies a parent body much larger than Vesta. It was long suspected that they were from Mars. Unfortunately there were two powerful arguments against that theory. Firstly there was the mathematics. Analysis of crater formation showed that to achieve escape velocity would require an impact of such high energy that all ejected material would melt. But the SNCs have not been melted. Secondly there was a statistical problem. It is far easier for an object to reach us from the Moon than it would be from Mars, but comparison of achondrites with samples from Apollo showed that there was not a single lunar meteorite.
The problem remained unsolved until 1982. In that year an Antarctic expedition discovered an unusual meteorite quickly recognised as being lunar. Several more have now been found, they are called Lunaites and have ages between 3.1 and 4.0Byr. A notable feature is that they have not been melted. This forced a re-think of the mathematics, which predicted that Lunaites, like SNCs, should have melted. It was found that a very low angle impact could indeed cause acceleration to escape velocity without melting, and that the impact would produce an elliptical crater. Such craters do exist on the Moon.
So that brings us back to the SNCs. They really are from Mars. The initials stand for Shergottites, Nakhlites and Chassignites, again all being similar to basalt. It is amazing that billions of dollars are being contemplated for a mission to Mars to bring back samples, yet we already have some!